The present invention relates to processes for depositing protective coatings. More particularly, this invention relates to a process for forming an improved bond coat of a thermal barrier coating system, such as of the type used to protect gas turbine engine components.
The operating environment within a gas turbine engine is both thermally and chemically hostile. Significant advances in high temperature alloys have been achieved through the formulation of iron, nickel and cobalt-base superalloys, though components formed from such alloys often cannot withstand long service exposures if located in certain sections of a gas turbine engine, such as the turbine, combustor and augmentor. A common solution is to provide turbine, combustor and augmentor components with an environmental coating that inhibits oxidation and hot corrosion, or a thermal barrier coating (TBC) system that thermally insulates the component surface from its operating environment. TBC systems typically include a ceramic layer (TBC) adhered to the component with a metallic bond coat that also inhibits oxidation and hot corrosion of the component surface.
Coating materials that have found wide use as TBC bond coats and environmental coatings include overlay alloy coatings such as MCrAlX where M is iron, cobalt and/or nickel and X is hafnium, zirconium, yttrium, tantalum, platinum, palladium, silicon or a combination thereof. Also widely used are aluminide coatings, which are generally single-layer oxidation-resistant layers formed by a diffusion process, such as pack cementation, above pack, vapor phase, chemical vapor deposition (CVD) or slurry coating processes. The diffusion process results in the coating having two distinct zones, the outermost of which is an additive layer containing an environmentally-resistant intermetallic represented by MAl, where M is iron, nickel or cobalt, depending on the substrate material. Beneath the additive layer is a diffusion zone comprising various intermetallic and metastable phases that form during coating as a result of diffusional gradients and changes in elemental solubility in the local region of the substrate.
Following deposition, the surface of a bond coat is typically prepared for deposition of the ceramic layer by cleaning and abrasive grit blasting to remove surface contaminants, roughen the bond coat surface, and chemically activate the bond coat surface to promote the adhesion of the ceramic layer. Thereafter, a protective oxide scale is formed on the bond coat at an elevated temperature to further promote adhesion of the ceramic layer. The oxide scale, often referred to as a thermally grown oxide (TGO), primarily develops from oxidation of the aluminum and/or MAl constituent of the bond coat, and inhibits further oxidation of the bond coat and underlying substrate. The oxide scale also serves to chemically bond the ceramic layer to the bond coat.
A bond coat is critical to the service life of the thermal barrier coating system in which it is employed, and is therefore also critical to the service life of the component protected by the coating system. During exposure to the oxidizing conditions within a gas turbine engine, bond coats inherently continue to oxidize over time at elevated temperatures, which gradually depletes aluminum from the bond coat and increases the thickness of the oxide scale. Eventually, the scale reaches a critical thickness that leads to spallation of the ceramic layer at the interface between the bond coat and the oxide scale. Once spallation has occurred, the component will deteriorate rapidly, and therefore must be refurbished or scrapped at considerable cost.
In view of the above, there is a continuous effort to improve the spallation resistance of TBC""s through improvements to the bond coat. Beneficial results have been achieved by incorporating oxides into the bond coat, as taught by U.S. Pat. No. 5,780,110 to Schaeffer et al. and U.S. Pat. No. 6,168,874 to Gupta et al., both commonly assigned with the present invention. Schaeffer et al. disclose inoculating the surface of a bond coat with a submicron dispersion of oxide particles that act as nucleation sites, thus reducing kinetic barriers to the formation of a desirable xcex1-alumina scale at the bond coat-TBC interface. The inoculated bond coat can be preoxidized to form a mature xcex1-alumina scale, or a TBC can be immediately deposited, during which the inoculated bond coat forms the desired mature xcex1-alumina scale. However, inoculating the bond coat surface prevents or at least limits the type of surface preparation that the bond coat can undergo prior to deposition of the TBC. For example, bond coat surface cleaning and roughening by grit blasting and electropolishing are precluded by the presence of the oxide particles at the bond coat surface. Gupta et al. avoid this complication by disclosing a method by which a diffusion bond coat and oxide particles are codeposited. However, Gupta et al. cannot readily control the types of oxides incorporated into their bond coat. Accordingly, other approaches for promoting the spallation resistance of a TBC through modification of its bond coat would be desirable.
The present invention generally provides a thermal barrier coating (TBC) system and a method for forming the coating system on a component designed for use in a hostile thermal environment, such as superalloy turbine, combustor and augmentor components of a gas turbine engine. The invention is particularly directed to a TBC system that exhibits improved spallation resistance as a result of having a bond coat formed to contain a dispersion of oxide particles in its outer surface region. A particular feature of this invention is the ability to preferentially entrap oxides of elements that are not present in the bond coat or the underlying substrate.
According to this invention, oxide particles are deposited on the surface of the component or an overlay coating deposited on the component surface, after which a diffusion aluminide bond coat is formed. Appropriate deposition of the bond coat causes the oxide particles to become dispersed in its outer surface region, e.g., limited to the additive layer of the diffusion aluminide bond coat. According to this invention, such a dispersion of entrapped oxide particles has been shown to significantly improve spallation resistance of a TBC deposited on a diffusion bond coat. The ability to selectively apply preselected oxide particles to a bond coat surface also provides performance and process advantages. For example, critical surface regions of a bond coat can be specially treated, and oxides of elements not present in the bond coat or substrate yet found to have a particularly beneficial effect can be readily and exclusively incorporated. In addition, this invention is applicable to both new components and those that require or have undergone localized repaired.
Other objects and advantages of this invention will be better appreciated from the following detailed description.